To understand how genome sequences relate to the biology of the organism, correct annotation of gene function is essential. There are many computational approaches to assigning a putative gene function based on similarity to known activities. However, as the evolutionary distances extend and the number
of putative homologs expands, the reliability of such unvalidated predictions becomes suspect. Likewise, while enzyme classes and broad activities may be predicted with some certainty, much less can be inferred regarding substrate specificity. This specificity defines the role of that enzyme at a functional level and understanding enzyme selectivity is essential to understanding the enzyme in the biology of the organism.